Intro to
Moles,
Formula
Mass
Chemistry 2/24/14
Drill
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•
•
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Make sure you have a calculator!!
What is the atomic mass of sodium?
What about sodium chloride?
Magnesium chloride?
• HW: p. 241 #26, 28, 30
Objectives
• IWBAT
– Calculate formula or molar mass.
– Define the mole and its relationship to
carbon-12.
– Convert from moles to grams, and back
again.
Formula Mass
• What is formula mass?
• The formula mass of any molecule,
formula unit, or ion is the sum of the
average atomic masses of the atoms
represented in the formula.
• What is a molecule? Formula unit? Ion?
Calculating Formula Mass
• Remember atomic mass?
– Atomic mass of Na = ?
– 22.99 u
– This is from the periodic table
• Now, we learn formula mass:
– What is the formula mass of NaCl?
– 22.99 u Na + 35.45 u Cl (mass of Cl)
– So, it is 58.44 u NaCl
What about other formulas?
• What’s the formula mass of nitrogen
tribromide?
• First, what’s the formula?
• NBr3
• Then, what should we do with the 3 bromines?
• 1 N atom x 14.01 u/N atom = 14.01 u
• 3 Br atom x 79.90 u/Br atom = 239.70 u
• Total = 14.01 u + 239.70 u = 253.71 u NBr3
Let’s practice formula mass!
1.
2.
3.
4.
5.
Na2SO3
H2SO4
MnO4C2H6O
Ca(NO3)2•2H2O
1.
2.
3.
4.
5.
126.05 u
98.09 u
118.94 u
46.08 u
200.14 u
What is a mole?
(or, What do small furry
creatures have to do with
Chemistry?)
Moles and Chemistry
• What do burrowing
mammals have to do
with Chemistry?
NOTHING!
• The mole in chemistry
is a unit of measure!
Counting Units
• What unit do you
use to measure
eggs or bananas?
• Dozen
• What unit do you
use to measure
shoes?
• Pairs
Other counting units
• Ream = 500
• Gross = 144
• Score = 20
So what?
• Let’s consider 6 Lego blocks for a
moment.
• Do all of these groups have the same
mass? Volume?
• How are they the same? How are they
different?
A definition
• A mole is a counting unit, based on the
number of atoms in exactly 12 grams of
carbon-12. This number is 6.022 x 1023.
• This is called Avogadro’s Number, after
Amelio Avogadro, an Italian scientist.
6.022 x
23
10 ?!
• Why do we need a counting unit that big?
• The mole is used for numbers of atoms and
molecules.
• BUT! Like all counting units, it can be used
for anything:
– A mole of pizzas, a mole of sheets of paper, a
mole of moles…
Atoms are very TINY!
• A drop of water the size of the period at the end
of a sentence on a piece of paper would contain
more than 10 trillion water molecules.
 Instead of talking about
trillions and quadrillions
of molecules (and more),
it's much simpler to use
the mole.
Molar Mass
• What is formula mass?
• Molar mass is the mass, in grams, of one
mole of a substance.
• Molar mass and formula mass are
numerically equivalent, but molar mass is
measured in grams per mole (g/mol).
State the number of moles
in 58.4 grams of Carbon.
• 12.01 g C = 1 mol C
• 58.4 g C = ? mol C
• 58.4 g C x 1 mol C = 4.86 mol C
12.01 g C
Formula Mass
• Let’s practice with Gram Formula Mass.
Wrapping up
• The smallest part of an ionic compound
is a
.
• The smallest part of a covalent
compound is a
.
• What is wrong with this question:
– Find the molecular mass of potassium
chlorate, KClO3.